Conventionally, detection and ranging systems that detect a target using radio waves (e.g., millimeter waves) have been put into practical use. The detection and ranging systems radiate a probe signal to a detection region and receive a reflected signal (i.e., an echo signal) from a target. By analyzing the received echo signal, the detection and ranging systems estimate a line-of-sight direction relative distance to the target (hereinafter simply referred to as “distance”), a line-of-sight direction relative speed of the target (hereinafter simply referred to as “speed”), and the direction to (or angle formed with) the target. Such detection and ranging systems include, for example, a plurality of receiving sensor elements arranged at equal spacing and, in accordance with, for example, the phase of an echo signal received by each receiving sensor element, estimates of various factors (such as distance, speed, and angle) for detecting the target.
In regard to the aforementioned detection and ranging systems, increasing the number of receiving sensor elements to enlarge an aperture of the receiving sensor array will increase the number of detectable targets, further enhancing, for example, the performance of angle estimation as well. Meanwhile, in the case of, in particular, a detection and ranging system installed in a vehicle or the like, the system needs to be downsized and the cost needs to be reduced.
In terms of downsizing of a system, a technology has been proposed wherein a configuration including a plurality of transmitting sensor elements and a plurality of receiving sensor elements is employed and an effective aperture of the receiving sensor array is widened by properly switching the sensors with a time sharing scheme. In this technology, however, for each time slot occupied by an individual transmitting sensor element, an echo signal vector needs to be obtained using the receiving sensor array, and the phase of a signal vector obtained at each time slot needs to be matched, requiring a long time to detect a target.
To solve this problem, the following detection and ranging method has been proposed. That is, probe signals spread at a plurality of modulators by using mutually orthogonal codes are radiated simultaneously (simultaneity is defined by, at least, defining the operation cycle of the system as a unit time; however, a plurality of unit times may be combined as a new unit time; this is also true for the following descriptions) from a plurality of transmitting sensor elements. Signals received by a plurality of receiving sensors are respectively split by a plurality of splitters, and the resultant signals are de-spread in demodulators using the same code as that used on the transmitter side. (e.g., Japanese Laid-open Patent Publication No. 2009-80024)
As other related arts, Japanese Laid-open Patent Publication No. 2010-243237 and Japanese Laid-open Patent Publication No. 2010-145289 are known.
A detection and ranging system employing a scheme of spreading a transmission wave by using a code is preferable in the sense that the system shortens the time required to detect a target. However, this kind of system needs, for example, a circuit for synchronous detection of a spread code, and hence the circuit size becomes large, increasing the production cost.